The instant invention pertains to measuring apparatus and more particularly to such apparatus which is used to measure the length of elongate material which is passed by the apparatus.
In the past, when it is necessary to measure a length of elongate material, e.g., cable being lowered into a well bore, various hand-held apparatus have been used which engage the cable by means of sheaves which are rotated as cable passes therethrough. Such rotation is used to generate a count on a mechanical counter in order to provide an indication of the length of cable unwound. Such devices have proved unsatisfactory in several respects. It is dangerous to hold such a device while cable is being wound off a large reel. Further, such devices have not proved to be entirely accurate.
Other apparatus for measuring cable length which incorporate a magnet on a rotating pulley have been proposed but have not proved satisfactory in all operations in which cable is lowered into a well bore. For example, after a well has been drilled and completed, and subsequent to a certain amount of production, it may be necessary to swab the bore in order to remove water from the well. Such swabbing is accomplished by lowering a line having jars suspended therefrom into the well bore. Such jars may be as long as twenty to thirty feet. Prior to lowering such jars into a well bore, a vertical pipe equal to the length of the jars, i.e., as long as twenty to thirty feet, is mounted on the well head in a vertical position on top of a valve which controls production from the well. The jars are received in the pipe which is sealed at the top prior to opening the well valve in order to permit lowering the jars into the well. It is difficult to accurately measure the length of cable being lowered into the well bore since such a measurement must be taken at the point at which the cable is unwound from its reel or at the point at which the cable enters the upright pipe which may be a distance of twenty to thirty feet above the ground.
Past measuring apparatus have proved to be unsuitable for use in measuring cable which is lowered into a well as described above.
One of the advantages of the instant invention is to permit accurate and safe measurement of a length of cable being lowered into a well bore in association with equipment as described above as well as in other configurations.
The instant invention includes a frame having a roller mounted for rotation on the top thereof. A second roller rotatably mounted on the frame has its circumference rollingly engaged with that of the other roller. A magnet is mounted on one of the rollers adjacent the circumference thereof and means for detecting a magnetic field is mounted on the frame adjacent the circumference of the roller having the magnet mounted thereon. When elongate material is passed between the rollers, they are rotated thus causing the detecting means to generate a pulse for each complete rotation of the roller having the magnet mounted thereon. The length of the material is proportional to the pulses so generated.